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Least Bell's vireo

Legal Status
Federal Endangered (1986)
State Endangered (1980)
Other The least Bell's vireo, Vireo bellii pusillus, is one of four
subspecies of Bell's vireo recognized in North America (Brown
1993).

Geographic Distribution
The historical distribution of least Bell’s vireo ranged from central
northern California through the Sacramento and San Joaquin valleys and
Sierra Nevada foothills, and from the southern Coast Ranges (including
the Santa Clara River watershed) to Baja California, Mexico (Wilbur
1979 and 1980 as cited in Kus 2002, USWFS 1998). Historical populations
were also documented in Owens Valley, Death Valley, and scattered
locations in the Mojave Desert (USFWS 1998, Kus 2002).

Today, the breeding range of least Bell’s vireo is limited primarily
from Santa Barbara County south to San Diego County (where the majority
of remaining populations occur) (Franzreb 1989, as cited in Labinger
and Greaves 2001a, Kus 2002). Breeding pairs have also been sighted
near Gilroy (Santa Clara County) (Roberson et al. 1997, as cited in Kus
2002) and along the Santa Clara River (Ventura County) (Labinger and
Greaves 2001a), Mojave River (San Bernadino County) (Kus and Beack
1998, as cited in Kus 2002), and San Joaquin River (San Joaquin County)
(River Partners 2005).

Critical habitat for the species has been designated in Santa Barbara,
Ventura, Los Angeles, San Bernardino, Riverside, and San Diego counties
(USFWS 1992). Critical habitat patches occur on the Santa Ynez, Santa
Clara, Santa Margarita, San Luis Rey, Sweetwater, San Diego, and
Tijuana rivers (USFWS 1992).

Local Distribution
A number of historical records document the presence of least Bell’s
vireo near and in the Santa Clara River watershed. One museum record
confirms the presence of a nesting pair of least Bell’s vireo in Foster
Memorial Park on the Ventura River in 1911 (CDFG 2005). Two least
Bell’s vireo observations were reported in 1980 and 1988: one 5.6 km
(3.5 mi) east of Piru and one at Newhall Ranch (CDFG 2005).

In 1990–1991, three separate records of least Bell’s vireo territorial
males and nesting pairs were reported near Saticoy and southwest of
Santa Paula (CDFG 2005). Multiple active least Bell’s vireo nests have
been observed on the Santa Clara River at the Fillmore Fish Hatchery
since 1991 (Labinger and Greaves 2001a, CDFG 2005). Greaves and
Labinger (1997) report capturing and banding 266 least Bell’s vireo
individuals between 1991 and 1996 in the lower half of the Santa Clara
River between I-5 downstream to the Highway 118 bridge near
Saticoy.

More recently, Labinger and Greaves (2001a) reported that least Bell’s
vireo was the most abundant and widely distributed endangered bird
species within the lower Santa Clara River area. Between 1994 and 1999,
they found 81 nesting pairs in the lower Santa Clara River, including
nine pairs between the McBean Parkway bridge (Santa Clarita, Los
Angeles County) and Las Brisas (Ventura County), 25 pairs from the
Fillmore Fish Hatchery downstream to the Highway 23 bridge, three pairs
from just downstream of the confluence with Sespe Creek to 2 km (1.2
mi) east of Santa Paula, 38 pairs along a 5 km (3.1 mi) segment
upstream of the Highway 118 bridge (including the Freeman Diversion),
and six pairs between the Highway 118 bridge and the Victoria Avenue
bridge (downstream of Highway 101, Ventura) (Labinger and Greaves
2001a). Breeding pairs were found at many of these same locations in
2000 (Labinger and Greaves 2001b). In 2003, there was a record of at
least one nesting pair on the Santa Clara River south of the Highway
101 bridge in 2003 (CDFG 2005).

The USFWS has designated critical habitat for the species along the
Santa Clara River between Ventura and Los Angeles counties, including
“all land within 3,500 feet perpendicularly and generally southward or
westward” of State Highway 126 between approximately Piru and
Interstate 5 near Castaic Junction (USFWS 1994).

Population Trends
Least Bell’s vireo is reported to have been common to abundant in its
historical range before undergoing a sharp decline in abundance and
range during the first half of the 20th century (USFWS 1998, Labinger
and Greaves 2001a, Kus 2002). USFWS (1998) report that:

By 1986, the population [of least Bell’s vireo] had declined to an
estimated 300 pairs, with the majority occurring in San Diego County.
Restoration efforts and brown-headed cowbird control have allowed
populations to increase in recent years. In 1998, the population size
was estimated at 2,000.

Based on population monitoring and compiled data covering 1991 to 2000,
Labinger and Greaves (2001a and 2001b) characterize the Santa Clara
River population as relatively stable to slightly increasing and with
increasing distribution along the river corridor. Between 1994 and
1999, Labinger and Greaves (2001a) documented 124 least Bell’s vireo
territories and a doubling of the vireo population from 25 to 57 pairs
in their original cumulative study area of 29 km (18 mi). In 1999,
after expanding their study area to include an additional 14 km (9 mi),
Labinger and Greaves (2001a) documented a total of 163 territories and
80 pairs. In 2000, they the documented 81 pairs in their expanded study
area (Labinger and Greaves 2001b). Count data from 18 points in the
lower 50 km (31 m) of the river reveal that the mean relative abundance
of least Bell’s vireo steadily increased from 0.28 in 1994 to 1.25 in
1998, before decreasing to 0.75 in 1999 (Labinger and Greaves 2001a).
In an upper reach of the river that had been affected by an oil spill,
the mean relative abundance of least Bell’s vireo decreased from 0.1 in
1994 to 0.03 in 1995 and then steadily returned to 0.1 in 1998 and 1999
(Labinger and Greaves 2001a).

Life History and Timing
The draft recovery plan for least Bell’s vireo (USFWS 1998) describes
the species as:

…a sub-tropical migrant, traveling some 2,000 miles annually between
breeding and wintering grounds. Preliminary results of studies of
color-banded birds indicate that least Bell’s vireos have a life span
ranging to seven years.

Least Bell’s vireos generally arrive in California from mid- to
late-March for a breeding season that typically ends in late September
(USFWS 1986, Kus 2002). During this period they are known to breed
almost exclusively within riparian habitats (USFWS 1998). Least Bell’s
vireos have been documented to return to the same breeding site year
after year (Greaves 1989).

According to the U.S. Fish and Wildlife Service (USFWS) (1998):

Males establish and defend territories through counter-singing, chase
and sometimes physical combat with neighboring males. Territory size
ranges from .20 ha to 3.03 ha (0.5 to 7.5 acres). … Newman (1992)
investigated the relationship between territory size, vegetation
characteristics, and reproductive success for populations of vireos at
the San Diego and Sweetwater Rivers, but found no significant factors
which could account for the variability in territory size observed at
his sites.

Nests are built by both breeding pair members within a few days of pair
formation, and generally take between four and five days to complete
(Kus 2002). Typically three to four eggs are laid beginning a day or
two after nest completion (Kus 2002). Both male and female share in egg
incubation, although females incubate more than males during the day
and appear to be the exclusive incubator at night (USFWS 1998 Kus
2002). Incubation lasts about 14 days, and nestlings fledge 10–12 days
after hatching (USFWS 1998, Kus 2002).

The fledgling stage is described in USFWS (1998):

Least Bell’s vireo may attempt as many as five nests in a breeding
season (B. Kus, pers. comm.), although most fledge young from only one
or two nests. … Adults continue to care for the young for at least two
weeks after fledging when territorial boundaries may be relaxed as
family groups range over larger areas. Fledglings generally remain in
the territory or its vicinity for most of the season, although the
behavior of older fledglings produced early in the year has not been
well studied.

Most vireo territories contain both dense vegetative cover within 1–2
meters of the ground, the preferred habitat for nesting, and a dense,
stratified overstory canopy, the preferred habitat for foraging
(Goldwasser 1981, USFWS 1998, Labinger and Greaves 2001a). In the Santa
Clara River watershed, Labinger and Greaves (2001) documented least
Bell’s vireo territories in early successional cottonwood/willow
forest, southwestern willow woodland, and mulefat scrub. While
vegetative structure was found to be more important in territory
selection than the presence of particular plant species, willow trees
and shrubs were found to be the most common plant species in the
vicinity of vireo territories and the preferred species for nest
placement (Labinger and Greaves 2001a).

Least Bell’s vireos have been observed to maintain territories that
include upland habitats adjacent to riparian areas, such as coastal
sage scrub (USFWS 1998). Upland habitats have also been documented for
foraging and for nesting when early spring floods inundate riparian
areas (Kus and Miner 1989, USFWS 1998). It has also been hypothesized
that berry-producing upland vegetation, such as laurel sumac (Malosma
laurina) and elderberry (Sambucus mexicana), may supplement the vireo
diet in marginal habitats (Kus and Miner 1989).

Nesting Habitat
Least Bell’s vireo primarily nests in small remnant segments of
vegetation typically dominated by willows (Salix spp.) and mulefat
(Baccharis salicifolia) but may also use a variety of shrubs, trees,
and vines (Olsen and Gray 1989). Nests are typically built within one
meter (3.3 ft) of the ground in the fork of understory vegetation
(Franzreb 1989, as cited in Kus 2002). Cover surrounding nests is
moderately open midstory with an overstory of willow, cottonwood
(Populus sp.), sycamore (Platanus sp.), or oak (Quercus sp.). Crown
cover is usually more than 50 percent and contains occasional small
openings. On the Santa Clara River, Labinger and Greaves (2001a) found
that the dominant plant species used for nest support (in 57 percent of
observed nests) were willows (Salix lasiolepis, S. exigua, S.
laevigata, and S. lasiandra), followed by mulefat (28 percent of
nests). The remaining 11 percent of nests observed were scattered
throughout a variety of tree, shrub, and forb species, including poison
oak (Toxicodendron diversilobum), white alder (Alnus rhombifolia),
mugwort (Artemisia douglasiana), and Fremont cottonwood (Populus
fremontii). Vireo’s were also found nesting in two invasive, non-native
plants: four percent of nests observed were found in giant reed (Arundo
donax) stands and two of the 426 nests observed were found in tamarisk
(Tamarix sp.) plants (Labinger and Greaves 2001a).

Foraging Habitat
Kuss (2002) indicates that the vireo typically forages in riparian and
adjoining upland habitat. Grinnell and Miller (1944) indicated that
foraging occurs at all levels of the canopy, but appears to be
concentrated in the lower to mid-strata, particularly when pairs have
active nests. Salata (1983) found that 69 percent of 131 foraging
observations were within 4 meters (12 feet) of the ground. Miner (1989)
found a similar peak in foraging activity in vegetation between 3–6
meters (9–18 feet) in height. Moreover, she determined that the
distribution of vireo foraging time across all heights was not simply a
function of the availability of vegetation at those heights, but rather
represented an actual preference for the 3–6 meter zone (Miner
1989).

Ecological Interactions
Least Bell's vireos are insectivores, preying on a wide variety of
insect types including bugs, beetles, grasshoppers, moths, and
particularly caterpillars (Chapin 1925; Bent 1950). It is likely that
vireos do not require water for drinking (Kus 2002). They obtain prey
primarily by foliage gleaning (picking prey from leaf or bark
substrates), and hovering (removing prey from vegetation surfaces while
fluttering in the air). Both Salata (1983) and Miner (1989) observed
vireos occasionally capturing prey by hawking (pursuit and capture of
flying prey), and Miner (1989) noted a behavior she called "clinging,"
which she described as hovering, but with the feet in contact with the
vegetation.

The invasion of exotic plant species into riparian habitats increases
habitat fragmentation and can decrease suitable vireo nesting habitat
in some cases. Invasive non-native plants found in current least Bell's
vireo habitat include castor bean (Ricinus communis), cocklebur
(Xanthium strumarium), tamarisk, and giant reed (USFWS 1998). Giant
reed is of prime concern due to its ability to disperse throughout an
entire drainage and its rapid growth rate, which allows it to
outcompete and restrict the growth of other native riparian plants (Kus
2002). When natural riparian vegetation types, such as the structurally
diverse native riparian scrub and mature forest communities required by
the vireo, are replaced by thick stands of giant reed, bird species
abundance and other native wildlife have been found to decline (Bell
1994, Bell 1997, Herrera and Dudley 2003, Kisner 2004). Labinger and
Greaves (2001a) observed over the course of their study (1994–1999)
that while dense thickets of giant reed supported very low bird
diversity:

… a low to moderate mixture of giant reed with native willow woodland
supported high bird diversity in some areas [such as near the Freeman
Diversion]…. In such areas giant reed was also used for nesting, as
noted by at least 17 nests of least Bell’s vireo, one nest of
southwestern willow flycatcher, and several other species such as
Anna’s hummingbird, bushtit, and common yellowthroat.

The ecological interaction of most concern to least Bell’s vireo
populations is brood parasitism by brown-headed cowbirds (Molothrus
ater). Dudek and Associates (2000) provide a description of brood
parasitism:

Cowbirds lay their eggs in the nests of other songbirds. The cowbird
often removes a number of the host’s eggs and replaces them with an
equal number of cowbird eggs. Cowbird eggs require a relatively short
incubation period, thus the young cowbird hatches earlier than the
host’s eggs. The effects of parasitism include reducing nest success
rate and egg-to-fledgling rate and delaying successful fledgling. A
common response to parasitism is abandonment of the nest. The success
rate of re-nesting is often reduced and there may be inadequate time to
prepare to migration. In California, parasitism rates range from 50
percent to 80 percent, considered to be a high parasitism rate.

USFWS (1998) describe least Bell’s vireo as a common host species that
readily accepts cowbird eggs. In the Santa Clara River watershed,
Labinger and Greaves (2001a) documented cowbird parasitism of vireo
nests and suggest that such parasitism may be limiting productivity of
host species. Labinger and Greaves’ (2001a) study “…did not find a
significant correlation between cowbird abundance and vireo
productivity. Parasitism rates of subpopulations of least Bell’s vireos
were never more than 20 percent and typically less than 10 percent.”
Removal of cowbird eggs and chicks from vireo nests during nest
monitoring has been cited to enhance vireo productivity by up to 44
percent in some studies (USFWS 1998).

Sensitivity to Anthropogenic Watershed Disturbances
Least Bell’s vireo is sensitive to the direct loss and degradation of
habitat and increased rates of cowbird parasitism that result from or
are exacerbated by urbanization and other development within and near
riparian areas. Urbanization and agriculture, including runoff from
agricultural fields and roadways, livestock grazing, water diversion
projects, traffic noise, feral pets, and recreational use of habitat
can result in the direct loss of vireo habitat, and degrade and
fragment habitat to the extent that it is no longer usable or increases
the vulnerability of the population. Least Bell's vireo often nest near
recreational open spaces or trails. Nest failure and abandonment can be
caused by human disturbance such as trampling of nests or nest sites or
clearing of vegetation (USFWS 1998).

Habitat fragmentation is thought to be one of the primary factors
responsible for vireo population decline, has been attributed to
development within riparian areas and the establishment and spread of
non-native plant species. Habitat fragmentation results in smaller
populations spread out among remaining suitable patches. These smaller,
more isolated populations then become more vulnerable to habitat
destruction (through flooding or development, for example), disease,
low production years, and parasitism (USFWS 1998, Labinger and Greaves
2001a).

The abundance of brown-headed cowbirds is believed to increase in areas
with development near riparian areas (USFWS 1998). Brood parasitism by
brown-headed cowbirds is the other primary factor, in addition to
habitat fragmentation, responsible for the decline of least Bell’s
vireo (Kus 2002, Labinger and Greaves 2001a).